Nice that you're gonna update it.
So, by thematic art, I mean choosing some specific themes to build upon. Perhaps having a futuristic theme? Then the scenario could be made of iron, the player objects could be robots, the enemies could be enemy robots, etc
Or perhaps the game happens on a mountain? Then the scenario would be pure ground and rocks, the characters could be animals which live on the mountains (one inside a ball and the other on top of the rocket?)
Things like that